The present invention relates to the formation of boules of semiconductor material from a melt of such material and, more particularly, to an improved method and means for controlling the replenishment of molten material in a crucible from which, for example, monocrystalline boules of material are grown by the Czochralski process.
The growing of large, solid monocrystalline boules of a semiconductor material is of major importance in the production of semiconductor devices. Such monocrystalline boules are sliced into thin wafers which provide the substrate on which integrated circuitry is formed. Silicon is the most widely used semiconductor material from which boules are grown.
It is generally considered relatively expensive to form silicon wafers. For one thing, the monocrystalline boules from which they are formed must be grown under carefully monitored and controlled conditions. This expense adds not only to the cost of integrated circuitry, but also has inhibited the use of silicon wafers for other purposes, e.g., the photovoltaic conversion of solar energy directly to electricity.
It has been the general practice in the field to grow monocrystalline boules of silicon in a discontinuous batch process. That is, most furnaces now in commercial use for this purpose typically include a single growth crucible containing a doped melt of silicon from which a boule is grown. The growth crucible is most often made of quartz so as to be relatively unreactive with the melted silicon contained by the same, and the crucible and its contents are heated to, and maintained at, a relatively high temperature, e.g., over 1400.degree. C., during the growth process. After a boule is completely grown, the quartz crucible and the remaining silicon within it are allowed to cool so that another charge of silicon can be placed in the furnace for the growth of a succeeding boule. This cooling typically results in the crucible breaking or cracking because of the different thermal properties of silicon and the quartz of the crucible. In other words, a new crucible has to be installed in the furnace for the growth of each boule. The crucible cost and down-time associated with its replacement are major contributors to the overall expense of growing monocrystalline silicon boules.
Because of the above problems, significant attention has been given to the development of commercially viable furnace arrangements which enable a plurality of boules to be sequentially grown from a single growth crucible. One approach under consideration is the recharging of a growth crucible from a separate, replenishment crucible to which solid polycrystalline material can be added and melted. This replenishment of melted material from a separate crucible can either be accomplished continuously or intermittently as the need warrants. U.S. Pat. No. 4,036,595 and copending U.S. Patent Applications Ser. Nos. 082,640 and 083,169 now U.S. Pat. No. 4,282,184 issued Aug. 4, 1981, all of which are owned by the assignee of this application, exemplify the state-of-the-art on this approach.
It is important in any arrangement in which a growth crucible is replenished by liquid flow from another crucible, that the flow of melted material from the replenishment crucible to the growth crucible be closely controlled. When the replenishment is accomplished during the growth of a boule in the growth crucible, e.g, when it is being accomplished continuously, it is important that the addition of replenishment melt not disturb the surface of the melted material in the growth crucible. And when a growth crucible is being recharged with melted silicon between the growth of boules, the rate of flow of the material must be closely controlled to prevent splashing and the like.
While the advantages of replenishment of silicon as a liquid melt from a separate replenishment crucible are apparent, so is one disadvantage. The provision of a separate replenishment crucible for a crystal growth furnace adds to the furnace capital cost due to the duplication of parts inherent in the provision of two separate crucibles. Besides the crucible duplication, itself, separate heating structures, height adjustment and rotational motive means, etc., must be provided for each.